Early magnetic tape storage devices moved the magnetic tape past read heads at a fixed velocity. A low pass filter is provided within the decoding circuitry that removes unwanted high frequency noise. Because the tape passes the read head at a fixed velocity a signal produced by reading data read from the tape has a knowing maximum frequency. Therefore, the cut off frequency of the low pass filter can be set to ensure all of the signal passes the filter.
It is now known to produce magnetic tape storage devices in which the velocity of the tape past the read bead is varied. Varying the velocity in this manner helps to ensure that the rate of data transfer to and from the tape can match the rate of data transfer to the storage device. This matching of data rates helps to prevent unnecessary stopping of the storage device. Stopping causes wear to the drive mechanisms and therefore, wear can be reduced if the drive can be slowed rather than stopped.
However, by altering the velocity at which the tape passes the read head, the frequency of the signal produced on reading data from the tape is altered. Therefore, it is desirable that the cut-off frequency of the low pass filter is altered accordingly. It is undesirable to have the cut-off frequency set too far above the maximum frequency of the signal since noise will not be effectively removed. Further, if the cut-off frequency is set too low then a portion of the signal will be lost. Generally, as the tape speed increases, data rate increases and it is necessary to increase the level of the cut-off frequency in order that the higher frequency data are not filtered or attenuated.
Prior solutions to this problem are known and an example is shown in FIG. 1. In this example a Phase Locked Loop (PLL) is used to lock onto the clock derived from the tape velocity. Control currents used in the PLL to adjust internal analogue parameters such that the PLL locks to the clock frequency are also fed to the filter. These control currents cause the cut off frequency of the low pass filter to be set at the correct position for the particular clock frequency.